1. Field of the Invention
The present invention relates to a complex boride cermet having a hard phase composed of a nickel-molybdenum complex boride and a complex boride cermet having a hard phase composed of a nickel-molybdenum complex boride with a part of the molybdenum substituted by tungsten. Particularly, it relates to a complex boride cermet having high strength, toughness, and thermal shock resistance, and the high strength is maintained even at elevated temperatures.
2. Discussion of Background
As a representative cermet which is practically used and enjoys a large market share, the cemented carbide (WC-Co cermet) may be mentioned.
This cermet is one of rare cermets practically used among a number of cermets so far studied.
For the cemented carbide (WC-Co cermet), many applications have already been established by virtue of its excellent properties such as high strength and high hardness.
However, it has a weak point such that when it is heated in atmospheric air to a temperature of 500.degree. C., tungsten carbide (WC) will be oxidized, whereby the strength decreases.
Whereas, a metal boride has a high melting point, high hardness and excellent corrosion resistance and oxidation resistance at high temperatures, and it is a good conductor of electricity and heat. Therefore, to utilize such properties of the boride, its application to e.g. mechanical parts where heat resistance and abrasion resistance are required, has been attempted with ceramics of the boride.
Especially, with respect to diboride ceramics such as titanium boride (TiB.sub.2) or zirconium boride (ZrB.sub.2), extensive research has been conducted (Journal of Japan Metal Association, 25, (12), 1081, 1986). Some of them have been practically used.
However, these borides are hardly sinterable materials, whereby it is difficult to obtain dense sintered bodies by a usual sintering method (pressureless sintering). (Hibata, Hashimoto, Quaternary Journal of Osaka Kogyo Gijytsu Shikenjo, 18, 216, 1967)
Whereas, it has been proposed to obtain a dense sintered body by using a sintering additive (Watanabe, Ishibai Powder and Powder Metallurgy, 26, 304, 1979) or by using hot pressing, and it has been made possible to obtain a sintered body having a density of almost 100%. However, for its application to mechanical parts or the like, such sintered body is still inadequate in the strength or toughness.
On the other hand, it has been proposed to bind such hardly sinterable boride with a matrix of a metal phase to obtain a complex material (cermet) wherein the properties of the boride are utilized (Kinoshita, Kose, Hamano, Journal of Ceramic Association, 75, 84, 1967, and Y. Yuriditskii et al, Poroshkovaya Metalluegiya., No. 4, (232), 32, 1982).
In this case, a dense sintered body is obtainable by a usual pressureless sintering method. However, from the viewpoint of strength, the product is still unsatisfactory.
The reason may be explained as follows.
Namely, the matrix of a metal phase which is expected to provide toughness, preferentially reacts with the boride and is converted to a brittle boride. For example, iron is converted to Fe.sub.2 B or FeB.sub.12, and Ni is converted to Ni.sub.2 B, Ni.sub.4 B.sub.3 or NiB, whereby the sintered body tends to be brittle.
Japanese Examined Patent Publication No. 15773/1981 (applicant: Toyokohan K.K.) proposes a high strength complex boride cermet to solve this problem. However, also in this case, the metal phase matrix is an iron base, whereby there are some problems in the corrosion resistance or oxidation resistance at high temperatures, and the properties of borides are not adequately utilized, particularly with respect to the strength at high temperatures. With respect to the phase relation of a Ni-Mo-B system, there has been a report by P. T. Kolomytsev and N. V. Moskaleva (Poroshkovaya Metalluegiya, No. 8, (44), 86, 1966). It has been reported that there exists a complex boride crystal phase of a tetragonal system having a composition of Mo.sub.2 NiB.sub.2 and a nickel alloy phase containing molybdenum.